using await in an expression generates invalid LLVM IR

[andrewrk]

const std = @import("std");
const expect = std.testing.expect;

test "await in expression" {
    _ = async atest();
}

fn atest() void {
    var f1 = async add(1, 2);
    var f2 = async add(3, 4);

    const sum = (await f1) + (await f2);
    expect(sum == 10);
}
fn add(a: i32, b: i32) i32 {
    return a + b;
}

Expected to pass. Instead:

Instruction does not dominate all uses!
  %21 = load i32, i32* %2, align 4, !dbg !1003
  %33 = call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %21, i32 %32), !dbg !1006
LLVM ERROR: Broken module found, compilation aborted!

Implementation needs to spill the intermediate values.